The present invention relates to the fabrication of integrated circuits. More particularly, the present invention is directed toward a method and apparatus for depositing titanium films on a semiconductor substrate.
Titanium containing films have many desirable characteristics for use in semiconductor processes. For example, titanium can function as a diffusion barrier between adjacent layers of differing material and prevent migration of one atomic species, or titanium containing films may serve to improve the adhesion between such layers. Titanium containing films may also be used to construct ohmic contacts by forming an alloy with semiconductor material.
Physical Vapor Deposition (PVD) systems were early employed to deposit titanium containing films on substrates. In an exemplary PVD system, a target consisting of a plate of titanium is connected to a negative DC voltage or radio frequency (RF) generator. A substrate holder facing the target is either grounded, floating, biased, heated, cooled, or some combination thereof. A gas, such as argon, is introduced into the PVD system, typically maintained at a pressure between a few millitorr (mtorr) and about 100 mtorr, to provide a medium in which a glow discharge can be initiated and maintained. When the glow discharge is started, positive ions strike the target, and titanium atoms are removed by momentum transfer. These titanium atoms subsequently condense into a thin titanium containing film on the substrate. However, the present dimensions of semiconductor devices has limited the usefulness of PVD. Many of these semiconductor devices have aspect ratios which frustrate deposition of uniform conformal layers due to shadowing effects.
Another technique to deposit titanium containing films on a substrate employed a conventional chemical vapor deposition (CVD) system. In an exemplary CVD system, reactive gases and diluent inert gases are flowed into a reaction chamber and gas species reactants are adsorbed on a silicon wafer substrate. Loosely bonded atoms migrate across the substrate and cause film-forming chemical reactions. The gaseous by-products of the reaction are then desorbed and removed from the reaction chamber. The chemical reactions that lead to formation of a solid material, such as a titanium containing film, may be either heterogenous, i.e., on the wafer surface, or homogeneous, i.e., in the gas phase. CVD systems include either hot wall reactors, i.e., reactors reaching temperatures of 600.degree. C. and greater, or cold wall reactor, reactors reaching temperatures less than 600.degree. C. Although hot wall reactors produce higher quality films, deposition on the walls often occurs which reduces the deposition rate, compared to the deposition rate of cold wall reactors for a given flow of process gases.
Increased deposition rates with suitable film quality has been achieved employing a plasma-enhanced chemical vapor deposition (PECVD) system. As is well known, a plasma, which is a mixture of ions and gas molecules, may be formed by applying energy, such as radio frequency (RF) energy, to a process gas in the deposition chamber under the appropriate process conditions, e.g., chamber pressure, temperature, RF power, and others. The plasma reaches a threshold density to form a self-sustaining condition, known as forming a glow discharge (often referred to as "striking" or "igniting" the plasma). This RF energy raises the energy state of molecules in the process gas and forms ionic species from the molecules. Both the energized molecules and ionic species are typically more reactive than the process gas, and hence more likely to form the desired film. Advantageously, the plasma also enhances the generation of reactive species in the gas phase to be deposited on the surface of the substrate allowing the formation of a better quality film at lower temperatures.
U.S. Pat. No. 5,173,327 to Sandhu et al. discloses a PECVD system in which the vapor of a heated liquid titanium source enters a modified, plasma enhanced, cold wall reaction chamber and is mixed with hydrogen as it reaches a wafer substrate surface. As the gas vapors reach the heated wafer substrate a chemical reaction of TiCl.sub.4 +2H.sub.2.fwdarw.Ti+4HCl occurs, thereby depositing a uniform titanium film upon the substrate surface. A drawback with prior art PECVD systems is that the same, if employing aluminum heaters, are subject to premature failure due to corrosion of the aluminum components contained therein.
In addition to corrosion of aluminum components, unwanted metal deposition is experienced with PECVD systems employed to deposit titanium containing films. Although the greatest film deposition generally occurs in places where the temperature is the highest, some deposition occurs at lower temperatures, resulting in unwanted deposition. Unwanted deposition can cause multiple problems, such as uneven deposition, arcing, degraded operation of chamber components, and/or device defects. Unwanted deposition may occur on non-conductive components, such as ceramic spacers and liners within the deposition chamber or chamber exhaust path, which then become conductive. This undesired conductive deposition can disrupt the shape of the glow discharge, resulting in uneven deposition across the substrate. It can also cause arcing, which may damage the substrate and parts of the chamber, such as the faceplate.
Further, titanium may build up on parts of the heater, in gas or vacuum apertures to undesirably restrict the flow therethrough, or on mechanical parts having close tolerances to interfere with their operation. Unwanted deposits that have bonded poorly to the underlying chamber component or that have built up on the heater may result in flakes and other particles that fall onto the substrate and cause defects, thus reducing substrate yield.
U.S. Pat. No. 5,508,066 to Akahori discloses a LPCVD system that reduces the aforementioned problems. Specifically, as disclosed therein, a plasma generation chamber is disposed adjacent to a reaction chamber. The plasma generation chamber is separated from the reaction chamber by a diaphragm which includes a centrally located plasma extraction window. A flow of gas is introduced into the reaction chamber that includes TiCl.sub.4 gas. A flow of argon and hydrogen gases is introduced into the plasma chamber. In this fashion, titanium residue in the plasma generation chamber is reduced. The process conditions of the system are then maintained at suitable parameters to form, upon a substrate disposed in the reaction chamber, a titanium containing film. However, the bifurcated processing chambers of the ECR system makes the system relatively expensive, as compared to PECVD systems, resulting in increased production costs of semiconductor devices manufactured therewith.
What is needed is a method and an apparatus for depositing a titanium containing film employing a single process chamber semiconductor processing system having deposition rates comparable with existing PECVD systems while avoiding unwanted deposition within the process chamber.